777 research outputs found
MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration
We discuss acceleration measurements for a large sample of extragalactic
radio jets from the MOJAVE program which studies the parsec-scale jet structure
and kinematics of a complete, flux-density-limited sample of Active Galactic
Nuclei (AGN). Accelerations are measured from the apparent motion of individual
jet features or "components" which may represent patterns in the jet flow. We
find that significant accelerations are common both parallel and perpendicular
to the observed component velocities. Parallel accelerations, representing
changes in apparent speed, are generally larger than perpendicular acceleration
that represent changes in apparent direction. The trend for larger parallel
accelerations indicates that a significant fraction of these changes in
apparent speed are due to changes in intrinsic speed of the component rather
than changes in direction to the line of sight. We find an overall tendency for
components with increasing apparent speed to be closer to the base of their
jets than components with decreasing apparent speed. This suggests a link
between the observed pattern motions and the underlying flow which, in some
cases, may increase in speed close to the base and decrease in speed further
out; however, common hydro-dynamical processes for propagating shocks may also
play a role. About half of the components show "non-radial" motion, or a
misalignment between the component's structural position angle and its velocity
direction, and these misalignments generally better align the component motion
with the downstream emission. Perpendicular accelerations are closely linked
with non-radial motion. When observed together, perpendicular accelerations are
usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa
Multiwavelength Observations of the Gamma-Ray Blazar PKS 0528+134 in Quiescence
We present multiwavelength observations of the ultraluminous blazar-type
radio loud quasar PKS 0528+134 in quiescence during the period July to December
2009. Significant flux variability on a time scale of several hours was found
in the optical regime, accompanied by a weak trend of spectral softening with
increasing flux. We suggest that this might be the signature of a contribution
from the accretion disk at the blue end of the optical spectrum. The optical
flux is weakly polarized with rapid variations of the degree and direction of
polarization, while the polarization of the 43 GHz radio core remains steady.
Optical spectropolarimetry suggests a trend of increasing degree of
polarization with increasing wavelength, providing additional evidence for an
accretion disc contribution towards the blue end of the optical spectrum. We
constructed four SEDs indicating that even in the quiescent state, the
bolometric luminosity of PKS 0528+134 is dominated by its gamma-ray emission. A
leptonic single-zone jet model produced acceptable fits to the SEDs with
contributions to the high-energy emission from synchrotron self-Compton
radiation and Comptonization of direct accretion disk emission. Fit parameters
close to equipartition were obtained. The moderate variability on long time
scales implies the existence of on-going particle acceleration, while the
observed optical polarization variability seems to point towards a turbulent
acceleration process. Turbulent particle acceleration at stationary features
along the jet therefore appears to be a viable possibility for the quiescent
state of PKS 0528+134.Comment: Accepted for Publication in The Astrophysical Journal. -
Acknowledgement adde
The magnetic field structure in CTA 102 from high-resolution mm-VLBI observations during the flaring state in 2016-2017
CONTEXT: Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to understanding the crucial physical processes giving rise to jet formation, as well as to their extraordinary radiation output up to Îł-ray energies.
AIMS: We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm VLBI polarimetric observations, reaching an unprecedented resolution (âŒ50 ÎŒas). We also investigate the variability and physical processes occurring in the source during the observing period, which coincides with a very active state of the source over the entire electromagnetic spectrum.
METHODS: We perform the Faraday rotation analysis using 3 and 7 mm data and we compare the obtained rotation measure (RM) map with the polarization evolution in 7 mm VLBA images. We study the kinematics and variability at 7 mm and infer the physical parameters associated with variability. From the analysis of Îł-ray and X-ray data, we compute a minimum Doppler factor value required to explain the observed high-energy emission.
RESULTS: Faraday rotation analysis shows a gradient in RM with a maximum value of âŒ6 Ă 104⎠rad mâ»ÂČ and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7 mm EVPA orientation is different when the component is exiting the core or crossing a stationary feature at âŒ0.1 mas. The interaction between the superluminal component and a recollimation shock at âŒ0.1 mas could have triggered the multi-wavelength flares. The variability Doppler factor associated with such an interaction is large enough to explain the high-energy emission and the remarkable optical flare occurred very close in time.Accepted manuscrip
Spatially resolved origin of mm-wave linear polarization in the nuclear region of 3C 84
We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultra-high angular resolution of 50ÎŒas (corresponding to 250R). We also add complementary multi-wavelength data from the Very Long Baseline Array (VLBA; 15 & 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0, and 343.5 GHz). At 86 GHz, we measure a fractional linear polarization of ~ 2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (~ 0.3-0.7% and < 0.1%, respectively). This suggests an increasing linear polarization degree towards shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ~ 10â”â»â¶ rad/mÂČ in the core at âł43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution, and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.Accepted manuscrip
MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets
We discuss the jet kinematics of a complete flux-density-limited sample of
135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to
investigate the structure and evolution of parsec-scale jet phenomena. Our
analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained
between 2002 and 2007, but includes our previously published observations made
at the same wavelength, and is supplemented by VLBA archive data. In all, we
have used 2424 images spanning the years 1994-2007 to study and determine the
motions of 526 separate jet features in 127 jets. The data quality and temporal
coverage (a median of 15 epochs per source) of this complete AGN jet sample
represents a significant advance over previous kinematics surveys. In all but
five AGNs, the jets appear one-sided, most likely the result of differential
Doppler boosting. In general the observed motions are directed along the jet
ridge line, outward from the optically thick core feature. We directly observe
changes in speed and/or direction in one third of the well-sampled jet
components in our survey. While there is some spread in the apparent speeds of
separate features within an individual jet, the dispersion is about three times
smaller than the overall dispersion of speeds among all jets. This supports the
idea that there is a characteristic flow that describes each jet, which we have
characterized by the fastest observed component speed. The observed maximum
speed distribution is peaked at ~10c, with a tail that extends out to ~50c.
This requires a distribution of intrinsic Lorentz factors in the parent
population that range up to ~50. We also note the presence of some rare
low-pattern speeds or even stationary features in otherwise rapidly flowing
jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal;
online only material is available from
http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi
Similarity of the Optical-IR and Gamma-Ray Time Variability of Fermi Blazars
We present the time variability properties of a sample of six blazars, AO
0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at
optical-IR as well as gamma-ray energies. These observations were carried out
as a part of the Yale/SMARTS program during 2008-2010 that has followed the
variations in emission of the bright Fermi-LAT-monitored blazars in the
southern sky with closely-spaced observations at BVRJK bands. We find the
optical/IR time variability properties of these blazars to be remarkably
similar to those at the gamma-ray energies. The power spectral density (PSD)
functions of the R-band variability of all six blazars are fit well by simple
power-law functions with negative slope such that there is higher amplitude
variability on longer timescales. No clear break is identified in the PSD of
any of the sources. The average slope of the PSD of R-band variability of these
blazars is similar to what was found by the Fermi team for the gamma-ray
variability of a larger sample of bright blazars. This is consistent with
leptonic models where the optical-IR and gamma-ray emission is generated by the
same population of electrons through synchrotron and inverse-Compton processes,
respectively. The prominent flares present in the optical-IR as well as the
gamma-ray light curves of these blazars are predominantly symmetric, i.e., have
similar rise and decay timescales, indicating that the long-term variability is
dominated by the crossing time of radiation or a disturbance through the
emission region rather than by the acceleration or energy-loss timescales of
the radiating electrons. In the blazar 3C 454.3, which has the highest-quality
light curves, the location of a large gamma-ray outburst during 2009 December
is consistent with being in the jet at ~18 pc from the central engine. This
poses strong constraints on the models of high energy emission in the jets of
blazars.Comment: Accepted for publication in ApJ. 15 pages (ApJ format), 6 tables, 11
figures. More discussion, explanation of analysis, and references have been
added. Major conclusions remain unchange
Disk-Jet Connection in the Radio Galaxy 3C 120
We present the results of extensive multi-frequency monitoring of the radio
galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as
well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of
observation, significant dips in the X-ray light curve are followed by
ejections of bright superluminal knots in the VLBA images. Consistent with
this, the X-ray flux and 37 GHz flux are anti-correlated with X-ray leading the
radio variations. This implies that, in this radio galaxy, the radiative state
of accretion disk plus corona system, where the X-rays are produced, has a
direct effect on the events in the jet, where the radio emission originates.
The X-ray power spectral density of 3C 120 shows a break, with steeper slope at
shorter timescale and the break timescale is commensurate with the mass of the
central black hole based on observations of Seyfert galaxies and black hole
X-ray binaries. These findings provide support for the paradigm that black hole
X-ray binaries and active galactic nuclei are fundamentally similar systems,
with characteristic time and size scales linearly proportional to the mass of
the central black hole. The X-ray and optical variations are strongly
correlated in 3C 120, which implies that the optical emission in this object
arises from the same general region as the X-rays, i.e., in the accretion
disk-corona system. We numerically model multi-wavelength light curves of 3C
120 from such a system with the optical-UV emission produced in the disk and
the X-rays generated by scattering of thermal photons by hot electrons in the
corona. From the comparison of the temporal properties of the model light
curves to that of the observed variability, we constrain the physical size of
the corona and the distances of the emitting regions from the central BH.Comment: Accepted for publication in the Astrophysical Journal. 28 pages, 21
figures, 2 table
A Connection Between Apparent VLBA Jet Speeds and Initial Active Galactic Nucleus Detections Made by the Fermi Gamma-ray Observatory
In its first three months of operations, the Fermi Gamma-Ray Observatory has
detected approximately one quarter of the radio-flux-limited MOJAVE sample of
bright flat-spectrum active galactic nuclei (AGNs) at energies above 100 MeV.
We have investigated the apparent parsec-scale jet speeds of 26 MOJAVE AGNs
measured by the Very Long Baseline Array (VLBA) that are in the LAT bright AGN
sample (LBAS). We find that the gamma-ray bright quasars have faster jets on
average than the non-LBAS quasars, with a median of 15 c, and values ranging up
to 34 c. The LBAS AGNs in which the LAT has detected significant gamma-ray flux
variability generally have faster jets than the nonvariable ones. These
findings are in overall agreement with earlier results based on nonuniform
EGRET data which suggested that gamma-ray bright AGNs have preferentially
higher Doppler boosting factors than other blazar jets. However, the relatively
low LAT detection rates for the full MOJAVE sample (24%) and previously known
MOJAVE EGRET-detected blazars (43%) imply that Doppler boosting is not the sole
factor that determines whether a particular AGN is bright at gamma-ray
energies. The slower apparent jet speeds of LBAS BL Lac objects and their
higher overall LAT detection rate as compared to quasars suggest that the
former are being detected by Fermi because of their higher intrinsic (unbeamed)
gamma-ray to radio luminosity ratios.Comment: 5 pages, 3 figures, 2 tables, accepted by the Astrophysical Journal
Letters; minor corrections to the text are mad
Detailed Structure of the X-ray Jet in 4C 19.44 (=PKS1354+195)
We investigate the variations of the magnetic field, Doppler factor, and
relativistic particle density along the jet of a quasar at z=0.72. We chose 4C
19.44 for this study because of its length and straight morphology. The 18
arcsec length of the jet provides many independent resolution elements in the
Chandra X-ray image. The straightness suggests that geometry factors, although
uncertain, are almost constant along the jet. We assume the X-ray emission is
from inverse Compton scattering of the cosmic microwave background. With the
aid of assumptions about jet alignment, equipartition between magnetic-field
and relativistic-particle energy, and filling factors, we find that the jet is
in bulk relativistic motion with a Doppler factor about 6 at an angle no more
than 10 degrees to the line of sight over deprojected distances about 150--600
kpc from the quasar, and with a magnetic field approximately 10 micro Gauss.Comment: To appear in "Black Holes: from Stars to Galaxies" Proceedings IAU
Symp. No. 238, eds. V. Karas & G. Matt, 2 pages, 1 figure, needs iaus.cl
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